1. Field of the Invention
The invention relates to vascular prostheses, and more particularly, to prostheses for endovascular repair of aortic aneurysms.
2. Description of Related Art
An aneurysm is a localized dilatation or weak spot in a blood vessel. Abdominal aortic aneurysms are one of the most common and serious types of aneurysm. They are thought to be atherosclerotic in origin, i.e., related to a high fat diet, high blood pressure, and cigarette smoking. But genetic factors that control collagen and/or elastic tissue metabolism may also be involved.
A common abdominal aortic aneurysm 10 is illustrated in FIG. 1. The aorta 12 is the large artery that arises directly from the heart. After it passes below the diaphragm, into the abdomen, the aorta 12 gives off major branches that supply the liver, stomach and spleen (the hepatic, left gastric and splenic arteries, the common trunk of which is the coeliac artery 14), the intestines (superior mesenteric artery 16), and the kidneys (renal arteries 18, 20). It then divides into right and left trunks 22, 24, called iliac arteries, which supply circulation to the legs.
Abdominal aortic aneurysms 10 most often develop in the relatively long segment 26 of aorta between the renal arteries 18, 20 and the bifurcation of the aorta into the right and left iliac arteries 22, 24. Abdominal aortic aneurysms 10 progressively enlarge at variable and unpredictable rates, and as they do, the involved aneurysm wall becomes weaker and thinner, and eventually ruptures. Rupture is relatively uncommon in abdominal aortic aneurysms less than five centimeters maximum transverse diameter, but the risk increases with increasing size. Rupture of abdominal aortic aneurysms causes approximately 15,000 deaths per year in the United States.
Surgery to prevent rupture is recommended for most patients with abdominal aortic aneurysms five centimeters maximum transverse diameter or greater. Presently approximately 40,000 of these operations are performed each year in the United States.
The standard operation involves a long midline incision in the abdominal wall that extends from the lower end of the sternum to the pubic bone. Blood flow through the aneurysm is temporarily stopped by occluding the aorta with a clamp placed above the aneurysm 10 and clamps placed across the right and left iliac arteries 22, 24, which are below the aneurysm. The aneurysm 10 is opened, and a plastic fabric tube graft is anastomosed (i.e., joined) to the normal, non-dilated aorta above the aneurysm 10. The other end of the tube graft is joined to the non-dilated aorta below the aneurysm 10.
Thus, the diseased segment (i.e., aneurysm) is replaced by a plastic tube. The anastomoses (i.e., junctions) between the aorta and the graft are sewn with surgical sutures. In some patients, a bifurcated graft in place of a single tube graft must be used. These grafts are shaped like an inverted "Y". The single trunk is joined to the proximal aorta and the right and left limbs are joined distally to the right and left iliac arteries (herein the words proximal and distal are used with the heart being the point of reference). These inverted Y grafts are used in patients who have no segment of non-dilated aorta below the aneurysm.
This major operation can be performed in relatively good-risk patients with a mortality rate of less than five percent. However, the operation requires five to ten days of postoperative hospital care and a four to six week recovery period. Furthermore, the operation cannot be safely performed in many patients with severe cardiac or pulmonary impairment.
Recently, alternative methods of repair that do not involve the large, open abdominal incision have been developed. One of these is the so-called "endovascular" approach. With the endovascular approach, a small incision is made in a groin to form a small opening in one of the iliac arteries 22. A guide wire 30 (FIG. 2) is passed up through the iliac artery 22 into the aorta 12 and through the aneurysm 10, under continuous X-ray, i.e., fluoroscopic control. A soft plastic fabric tube graft 29 is placed over a metal mesh stent 31 (in its non-extended state) which is mounted over the balloon portion 32 (non-distended) of a large angioplasty type catheter 34. The fabric graft is folded longitudinally so that the entire apparatus (i.e., plastic graft, underlying stent and balloon angioplasty catheter) can be loaded into a flexible plastic sheath 33 (e.g., Teflon.TM.) and passed into the lumen of the iliac artery 22 and up into the aneurysm 10. Once the prosthesis is properly positioned, the sheath 33 is withdrawn.
Proximal fixation of the graft is accomplished with the underlying stent. The upper end of the graft is secured by the stent to the segment of non-dilated aorta 36 just above the aneurysm 10. This segment 36 that lies above the aneurysm but below the renal arteries is sometimes referred to as the "neck" of the aneurysm (and will be referred to as such herein), although it is not actually a part of the aneurysm. After withdrawal of the delivery sheath 33, the balloon 32 is distended to expand the stent 31 so that the stent 31 presses the outer wall of the graft 29 against the inner wall of the neck 36 of the aneurysm. An alternative method of fixation involves a ring of interconnected, fine-metal hooks or barbs pre-sewn into the upper end of the graft. The metal hooks or barbs are "fired" into the wall of the neck of the aneurysm by inflation of the catheter balloon 32. Yet another known method is a combination of a stent and hooks or barbs, in which the hooks or barbs are welded to the stent.
Proper proximal fixation of the graft to the aorta is important for two reasons. One, proximal fixation prevents leaks to the outside of the graft, leaks which would maintain high arterial pressure within the abdominal aortic aneurysm and lead to progressive enlargement and rupture. Two, proximal fixation prevents distal migration and collapse of the soft plastic fabric tube graft by the force of arterial blood flow. Over time, fibrous tissue grows from the aortic wall into the interstices of the porous fabric graft providing some additional fixation.
Blood pressure and flow tend to keep the lower end of the graft expanded against the aortic wall by a so-called "parachute" effect. However, distal fixation of the graft is generally accomplished by placement of a second short, expandable metal stent similar to the stent used in the upper end. The second stent is expanded at the lower end of the graft so that the stent presses the outer wall of the graft against the inner wall of the aorta below the aneurysm.
There are several major unsolved problems with proximal fixation of endovascular grafts that limit their application in the repair of many abdominal aortic aneurysms. One such problem is that the aneurysm neck 36 is often too short to permit adequate fixation with an expandable stent. (Generally, a length of two centimeters or more is needed for adequate anchoring of the graft.) Additionally, the aneurysm neck is frequently too heavily calcified to permit fixation with hooks, i.e., the hooks cannot penetrate areas of the aortic wall that have thick, calcified plaques. In other cases, the inner wall of the neck is thickened by soft, friable plaque or thrombus that makes fixation with hooks impossible or inadequate.